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March 23, 2015
Clarifying the Particle Acceleration Mechanism through Shock Waves in a Plasma: Introducing Research Utilizing the Plasma Simulator

Plasma, which is a collection of charge-bearing particles, engages in movements that are extremely varied and complicated. In order to clarify the qualities of such plasma, large-scale numerical simulations performed by a supercomputer are indispensable. At the National Institute for Fusion Science is a supercomputer, the Plasma Simulator. This machine is deployed for comprehending and predicting the movement of plasma in the Large Helical Device (LHD). This simulator is used by researchers and graduate students based at universities throughout Japan for developing simulation methods and investigating the various complicated phenomena of plasma. Here, we will introduce one such type of simulation research.

The source of the Sun’s energy is a fusion reaction. The Sun is ejecting not only light and heat but also plasma into space. The corona, which is the atmosphere of the Sun’s surface, is plasma of approximately 1,000,000 degrees and in a condition in which electrons and ions are moving about. There occasionally occur flares, which are great releases of plasma. When a solar flare occurs, a large volume of plasma approaches near to Earth and causes auroras in the skies above the South Pole and the North Pole or power outages and disruptions in communication through disturbances in the magnetic field. Further, solar flares also produce electrons and ions of extremely high levels of energy (below as “high energy particles”). These particles sometimes reach energy levels that are 1,000,000 times greater than prior to the eruption of a solar flare. How this energy can be accelerated has not yet been clarified. Thus, we are investigating mechanisms for accelerating high energy particles by using the Plasma Simulator.

In generating high energy particles, it is thought that shock waves play an important role. On Earth, too, for example, shock waves are generated through powerful volcanic eruptions and other phenomena. When a meteorite struck Russia in February 2013, many glass windows were broken and other damage was caused by the shock wave. In the Sun, the shock waves generated by flares expand through the solar atmosphere composed of electrons and ions inside the corona. Then, when the electrons and ions meet the shock wave, they begin complicated and violent movement. Further, the electrical field and the magnetic field change due to the movement of those particles. Moreover, the electromagnetic field influences the movement of the particles. Through tracking the movement of the many particles and the behavior of the electromagnetic field, we have become able to reproduce the condition in which the shock waves inside a plasma accelerate the particles to high energy. A result of the simulation is that we can explain the energy of particles that are produced by a solar flare. In addition, we also have clarified the conditions under which the heavy ion, which bears the positive charge, and the light electron, which bears the negative charge, each is accelerated through different mechanisms, and the condition under which their acceleration occurs.

In the future, because we will further improve the performance of the supercomputer, we will develop calculation methods for deploying maximally that performance and for verifying particle acceleration through shock waves inside a plasma through even larger scale and higher precision simulations. Further, because high energy particles that are produced by solar flares damage instruments carried in artificial satellites, it is necessary to predict when those particles would reach Earth. And to improve that prediction ability, it is anticipated that clarification of the mechanism by which high energy particles are generated will advance further.